The present invention concerns high frequency inductive generator for the inductive coil of a plasma generator intended more particularly for analyses by emission spectrometry, to produce a plasma flame from a gas jet, with an aerosol of the solution to be analyzed being introduced into this flame.
There is a known method of obtaining a plasma flame consisting in introducing a jet of gas such as argon into an inductor coil supplied at very high frequency, of the order for example of 40 to 60 MHz. The high frequency generator is therefore a fundamental element of an apparatus for analysis by emission spectrometry with inductive plasma.
Such an HF generator must be able to be connected to a very high impedance during the short period between starting and the ignition of the flame, and must then adapt automatically to a low impedance on ignition, and therefore present a very high surge coefficient.
Such HF generators have been made, with an oscillator of the resonant line type for forming plasma, or for other applications necessitating the use of high frequencies. According to this technique, represented diagrammatically by FIG. 1, two identical triodes 1 and 2 are used whose quarter wave anode lines 3 and 4 are connected at 5 where the high voltage is applied, of the order for example of 3,000 to 5,000 volts. The cathode lines of the two tubes are also coupled, like the grids provided with a resistor 7 and with decoupling capacitors 8 and 9. The lines 10 for the inductor 11 are connected by a capacitor 12. To suppress the static component of the coupling and pick up only the magnetic component, a screen 14 formed by a series of open dipoles is disposed between the loop of the anode lines 3 and 4 and the loop of coupling lines 10. A more complete study of such a generator can be found in an article published in 1976, pages 271 to 282 in number 84 of the journal "Analytica Chimica Acta".
Such a high frequency generator works entirely correctly as long as the tubes 1 and 2 have identical characteristics, which can be obtained with new, matched tubes, or if they both age in entirely parallel ways. In practice, this is rarely the case, and the differential aging of the two tubes leads to unbalancing of the circuits, by displacement of the zero point of the high frequency which causes serious distances in the operation of the generator. Moreover, the bulk of the two tubes and their supplies and attached members quickly becomes excessive when significant power levels are concerned.